Why it all started

In the early 80s reports of soviet engineers designing a new MBT came in. These mbts would later be known as the T-80. However at the time other projects were also tested like the Object 477 which had more advanced armour compared to any other soviet tank at the time. Which in turn ment that the US would need to develop a new way of dealing with such tanks.

Worrying that the M1 Abrams might be outgunned by the soviets the US army launched the Armored Systems Modernization project, ASM for short. The ASM projects goal was to achieve undeniable battlefield dominance over the Soviet Union with a unified, modular family of heavy combat vehicles.

The birth of the 140mm

With the 120mm already being pushed to its limits in terms of armour penetrating capabilities the engineers started to think bigger. Thats when they came up with the ATAC System. The goal of the ATAC system project was to achieve complete the most firepower possible.

Thats why they increased the size of the calibre to 140mms. Altough the XM 291 cannon could still be made into an 120mm to take “old” ammuntion the most powerful variant of the XM 291 was the 140mm. It had almost double the muzzle energy of the standard 120mm M256, and with that it could penetrate up to 1200mm of armour at 1km range.

Due to the size (1.5m, 4,9 feet) and weight (39kg, 88lbs) of the shells the gun had to be autoloaded with the XM91 autoloader.

The CATTB enters the scene

This was the big moment for the engineers to throw every single new technology into one single tank. Building it of the abrams they made it longer to fit the extra ammo in the hull, gave it improved armour in the front, gave it an multi-fuel turbocharged diesel engine with 1450hp, hydropneumatic suspension. And of course the 140mm XM 291 with the turret along side it.

The CATTB was the centerpiece of the whole ASM project and in 1988 the most powerful and highly advanced tank ever built.

The rather unfortunate end

In the end the project came out to be way to expensive for it to become an reality, with 63 billion $ already spent. And with the huge success of the Abrams tank in Operation Desert storm and the collapse of the Soviet Union the US army thought to themselves “Why spend billions on a futuristic super tank project when our current tank is still more than good enough, besides the soviets wont develop a new tank anytime soon”.

Sources:

https://armoredwarfare.com/en/news/general/development-cattb

https://de.wikipedia.org/wiki/XM291_Advanced_Tank_Cannon

https://www.secretprojects.co.uk/threads/cattb%EF%BC%9Aa-weak-giant%EF%BC%9F.40920/

https://forum.warthunder.com/t/high-caliber-projects-discussion-and-implementation/11001